Gonadotropin-inhibiting hormone
(GnIH) stimulates food intake and inhibits reproduction. Neuropeptide Y (NPY) is also involved in appetite regulation and
reproduction. Here we investigated whether the effects of GnIH could be
mediated via actions on NPY neurons. Mice were used so that we could take advantage
of genetically modified animals in which the NPY neurons expressed GFP, enabling
their in vitro identification.

Coronal brain slices 250 mm thick and including the arcuate nucleus, were
cut and mounted on an upright microscope for patch-clamp electrophysiology
recordings. Slices were continuously superfused with artificial cerebrospinal
fluid (aCSF) at 32°C and
viewed with infrared DIC and epifluorescence optics. GnIH, 10 nM, 100 nM and 1 mM, was applied locally for 30s.

First, most NPY cells were inhibited by GnIH at all concentrations.
This was manifest by a reversible reduction in firing rate of spontaneous
action potentials together with hyperpolarization of the membrane. Current-voltage
relationships revealed that the effects of GnIH generally involved inhibition
of a mixed cation conductance and activation of a K+ conductance,
which explain the hyperpolarization and inhibition. Second, GnIH inhibited
secretion of NPY in incubated hypothalamic blocks, consistent with the
electrophysiology. Third, tetrodotoxin (TTX), to inhibit action potential
activity, had effects that were strikingly similar to those of GnIH, indicating
the possibility of an interneuron in the circuitry. Fourth, neither GniH nor
TTX had significant effects on spontaneous excitatory postsynaptic current frequency or amplitude, thus excluding presynaptic effects. Fifth,
immunohistochemistry revealed that only 20% of NPY cells had close GnIH
contacts.

Thus, GnIH has several actions on NPY neurons in mouse
arcuate nucleus, with inhibition being prominent. These effects may reflect
subpopulations of NPY neurons. The main effect of GnIH may be to inhibit an
excitatory input to NPY neurons, thereby inhibiting the activity of NPY neurons
in an indirect manner.